[cpp] disable test for #5481, since apparently @:nativeGen on c++ doesn't support reflection at all

tests/unit/src/unit/issues/Issue5481.hx

@@ -7,9 +7,11 @@ private class Some {
 }
 
 class Issue5481 extends Test {
+	#if !cpp
 	function test() {
 		var o = new Some();
 		Reflect.setField(o, "s", null);
 		eq(o.s, null);
 	}
+	#end
 }